(1) Field of the Invention
The present invention relates to a process for washing an electronic material such as a silicon wafer, especially a semiconductor substrate, with an organic solvent. More specifically, the present invention relates to a process for circulating an organic solvent in order to reuse it to make use of the organic solvent washing.
(2) Description of the Prior Art
When a semiconductor substrate is processed into an intended part of an electronic appliance, the semiconductor substrate is subjected to mechanical treatments such as cutting and grinding and chemical treatments such as chemical etching with an acid or the like. Cut chips, grounds, electrolytes such as acids and clusters of oily substances adhere to the processed substrate, and these adhering substances are removed by water washing. But even if ultra-pure water is used for this water washing, impurities such as the electrolyte fine particles of solid or clustery suspended oils (collectively referred to as "particles" hereinafter) and a high-boiling-point substance are contained in water drops adhering to the substrate after water washing, and if water is removed only by evaporation and drying, stains called "water marks" are formed on the semiconductor substrate and defective products are prepared.
Accordingly, after water washing, water and impurities contained in water must be removed by washing with an organic solvent, especially an organic solvent having a good compatibility with water. This operation is called "organic solvent washing" in the instant specification. As the solvent ordinarily used for the organic solvent washing, there can be mentioned alcohols such as ethanol, n-propanol, isopropanol, isobutanol and isopentanol, halogenated hydrocarbons such as methyl chloride, methylene chloride, carbon tetrachloride, trichloroethane, trifluoro-ethanol and perchloroethylene, ethers such as diisopropyl ether and ethylisobutyl ether, hydrocarbons such as hexane, cyclohexane and benzene, and esters such as isobutyl formate, propyl acetate and methyl propionate. An alcohol, especially isopropanol (hereinafter referred to as "IPA"), is popularly used.
If the same solvent is used repeatedly for the organic solvent washing, impurities such as water, electrolytes and particles are naturally accumulated in the solvent and the amounts of these impurities are increased. When the water concentration in the organic solvent used for the organic solvent washing is thus changed, the quality or yield of the semiconductor substrate is delicately affected by the water content, and hence, it is important to maintain the quality of the organic solvent per se stable. As means for attaining this object, there may be considered a method in which the organic solvent is frequently exchanged with fresh one. However, this method is defective in that the economical loss is large. Moreover, when the organic solvent is exchanged, impurities such as particles are incorporated into the organic solvent from a vessel or the circumference and then the fresh organic solvent is frequently contaminated.
Accordingly, in view of the stabilization of the quality and from the economical viewpoint, it is preferred that the once-used organic solvent containing various impurities (hereinafter referred to as "waste organic solvent") be purified and reused repeatedly.
However, the method in which the waste organic solvent is purified involves several problems. As is apparent from the intended use of the organic solvent, it is necessary that the organic solvent used for the organic solvent washing should have a certain water-dissolving power, preferably a water-dissolving power of at least 10% by weight, and the organic solvent has a large polarity and forms an azeotropic mixture with water. For example, isopropanol forms an azeotropic mixture with water at a concentration of about 88% by weight (the water concentration is 12% by weight). It is therefore impossible to concentrate an aqueous solution of IPA having a lower concentration beyond the concentration of 88% by weight by an ordinary distillation operation. There can be mentioned a method in which an entrainer such as benzene is added to an aqueous solution of IPA and azeotropic distillation is carried out. However, in carrying out this method, there should be arranged at least three columns, that is, the column for dehydration with the entrainer, the column for removing water recovered by the entrainer and the column for purification of IPA. And also in order to increase the organic solvent concentration to a level adopted for the organic solvent washing, that is, at least 99% by weight, preferably at least 99.5% by weight, a rectification column having at least 100 stages becomes necessary. This corresponds to a column height exceeding 6 m.
As the method for purifying the waste organic solvent which is in the solvent-rich state where the concentration of water incorporated by the organic solvent washing does not reach the azeotropic level, in case of, for example, IPA, there may be considered a method in which by utilizing the azeotropic distillation, water is removed in the form of an IPA-water azeotropic mixture from the head of the azeotropic distillation dehydration column and IPA having a reduced water content is recovered from the column bottom. According to this method, however, a rectification column having about 80 stages is necessary, and an azeotropic distillation dehydration column having a height of at least about 5 m is necessary. Furthermore, the loss ratio of IPA is ordinarily higher than 20%, and it is considered that this method cannot be put into practical use. Of course, if IPA is withdrawn from the column bottom, particles or electrolytes cannot be removed. Accordingly, after removal of the initial fraction having the azeotropic composition, distillation of IPA should be carried out, and since the purified solvent cannot be taken out while the initial fraction is being removed, continuous circulation is impossible.
There may also be considered a method in which a drier such as a molecular sieve is used as means for dehydrating the organic solvent. However, this method is not suitable for drying an organic solvent containing water in such a large amount as exceeding 10% by weight, and there is a risk of new incorporation of particles through the molecular sieve. Therefore, this method is not substantially preferred.
Under this background, it has been eagerly desired to develop a process for washing a semiconductor substrate with an organic solvent, which is provided with a purification system in which the waste organic solvent is effectively purified in a chamber such as a clean room with a reduced loss ratio without a risk of incorporation of impurities.